OF SOUTHERN INDIA. 3 



occasionally three lamellae on. each side ; and in some of the Arcid^ each gill is so 

 completely folded over on itself as to represent a double lamella. In general appear- 

 ance and structure the gills of the Pelecypoda do not differ in any way from those 

 of most other aquatic animals, and they perform the same functions of respiration. 



The water is supplied to the gills, either through a special siphon whicii is 

 situated posteriorly below the posterior muscular scar and is produced into a 

 longer or shorter tube, or it has access to the gills within the greater part of 

 the open space of the mantle. However, even in such forms as Astarte or Vulo, 

 in which the mantle-margins are below perfectly disunited, it Avill be observed 

 in the living animal that, as a rule, the water is admitted to the gills only, 

 or especially, at the place where the inhalant siphon should be situated; this 

 place is provided with elongated cilia and generally kept open, while on the ventral 

 side itself the mantle-margins fit closely against each other. The current of water 

 passes through the inhalant (lower) opening, or siphon, along the inner, lower, or 

 ventral, side of the animal and returns along the attached bases of the gills, issuing 

 from the internal cavity by the upper or exhalant opening, which is either grown 

 together with the inhalant siphon and equally prolonged, or separated from it, and 

 represented by a simple notch or slit in the mantle. Only in the CrassatellidjS, 

 some of the MYTILACEA, and in all the OSTUEACJEA, the anal, or exhalant, 

 ojoening is not specially marked. The continuous current is, however, not suffi- 

 cient to carry off the water which enters the body by the inhalant siphon, or 

 other openings of the mantle : In almost all Pelecypoda which I observed the 

 body appeared from time to time to contract, the animal generally closing the 

 posterior siphonal openings and ejecting with great force a quantity of water 

 through the pedal and ventral opening. This forcible ejection of the water also 

 often appears to be done with the object of assisting in locomotion. 



The foot is a retractile and expansile muscular mass situated on the anterior 

 ventral side ; it is variable in size and shape. The most usual form of the foot is 

 sub-cylindrical or somewhat broadly elongated, with the lower anterior edge 

 more or less distinctly sharpened, like that of a hatchet, and hence the name of the 

 entire class. In some families, as in the Solenidm, the foot is club-shaped, but it 

 has the bluntly sharpened edge in front ; in some of the Arcidm, Nuculidm, and 

 allied forms, the sole can be expanded and again folded together, so as to form a 

 sharp, generally serrated edge. In the Erycinid^ it is truncate and can be 

 dilated into a kind of disc; only in the Lucinid^, and a few species of the 

 Mytilxdm, it is vermiform, obtusely rounded or truncate at the end. Sometimes 

 there are at the posterior side of the foot certain horny fibres secreted for the 

 purpose of enabling the animal to attach itself permanently, or temporarily, to 

 solid' objects ; these fibres are called the bijssiis, and in the species which have it 

 developed the foot generally becomes more or less rudimentary, though it never 

 disappears. In some of the attached OSTREACEA, the foot, however, becomes 

 obsolete in the full grown animals, but it does exist in the very young ones. 

 Internally the foot is very often hollow and penetrated by an aqueous canal, 



